Positive electrode active material, non-aqueous electrolyte secondary battery and method for producing positive electrode active material

ABSTRACT

A non-aqueous electrolyte secondary battery employing a positive electrode active material containing a compound represented by the general formula Li x M y PO 4 , where 0&lt;x≦2 and 0.8≦y≦1.2, with M containing a 3d transition metal, where Li x M y PO 4  encompasses that with the grain size not larger than 10 μm. The non-aqueous electrolyte secondary battery has superior cyclic characteristics and a high capacity.

The present application is a divisional of U.S. patent application Ser.No. 09/701,903 filed Feb. 14, 2001 now U.S. Pat No. 6,632,566.

TECHNICAL FIELD

This invention relates to a method for producing a positive electrodeactive material that is capable of reversibly doping/undoping lithium,and a method for producing a non-aqueous electrolyte secondary batteryemploying this positive electrode active material.

BACKGROUND ART

Recently, with the marked progress in a variety of electronic equipment,researches in a rechargeable secondary battery, as a battery that can beused conveniently and economically for prolonged time, are underway.Typical of the known secondary batteries are a lead battery, an alkalistorage battery and a lithium secondary battery.

Of these secondary batteries, a lithium secondary battery has advantagesin high output and in high energy density. The lithium secondary batteryis made up at least of positive and negative electrodes, containingactive materials capable of reversibly introducing and removing lithiumions, and a non-aqueous electrolyte.

Nowadays, a compound having an olivinic structure, such as, for example,a compound represented by a general formula Li_(x)M_(y)PO₄, where x issuch that 0<x≦2 and y is such that 0.8≦y≦1.2, with M containing a 3dtransition metal, is retained to be promising as a positive electrodeactive material for a lithium secondary battery.

It has been proposed in Japanese Laying-Open Patent H-9-171827 to usee.g., LiFePO₄, among the compounds represented by Li_(x)M_(y)PO₄, as apositive electrode for a lithium ion battery.

LiFePO₄ has a theoretical capacity as high as 170 mAh/g and, in aninitial state, contains electro-chemically dopable Li per Fe atom, sothat it is a material promising as a positive electrode active materialfor a lithium ion battery.

Up to now, LiFePO₄ was synthesized using a salt of bivalent iron, suchas iron acetate Fe(CH₃COO)₂, as a source of Fe as a starting materialfor synthesis, and on sintering the starting material at a highertemperature of 800° C. under a reducing atmosphere.

However, it is reported in the above publication that, in the batteryprepared using LiFePO₄, prepared by the above method for synthesis, asthe positive electrode active material, the real capacity only on theorder of 60 mAh/g to 70 mAh/g may be realized. Although the realcapacity of the order of 120 mAh/g has been reported in Journal of theElectrochemical Society, 144, 1188 (1997), this real capacity cannot besaid to be sufficient in consideration that the theoretical capacity is170 mAh/g.

If LiFePO₄ is compared to LiMn₂O₄, LiFePO₄ has a volumetric density andan average voltage of 3.6 g/cm2 and 3.4 V, respectively, whereas LiMnPO₄has a volumetric density and an average voltage of 4.2 g/cm² and 3.9 V,respectively, with its capacity being 120 mAh/g. So, LiFePO₄ is smallerby approximately 10% in both the voltage and the volumetric density thanLiMn₂O₄. So, with the same capacity if 120 mAh/g, LiFePO₄ is smallerthan LiMn₂O₄ by not less than 10% in weight energy density and by notless than 20% in volumetric energy density. Thus, for realizing anequivalent or higher level in LiFePO₄ with respect to LiMn₂PO₄, acapacity equal to or higher than 140 mAh/g, is required, however, such ahigh capacity has not been achieved with LiFePO₄.

On the other hand, with LiFePO₄, synthesized on sintering at a highertemperature of 800° C., there are occasions where crystallizationproceeds excessively to retard lithium diffusion. So, with thenon-aqueous electrolyte secondary battery, sufficiently high capacityhas not been achieved. Moreover, if the sintering temperature is high,the energy consumption is correspondingly increased, while a higher loadis imposed on e.g., a reaction apparatus.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a positive electrodeactive material which realizes a high capacity if used in a battery, anda non-aqueous electrolyte secondary battery employing the positiveelectrode active material.

For accomplishing the above object, the present invention provides apositive electrode active material containing a compound represented bythe general formula Li_(x)M_(y)PO₄, where 0<x≦2 and 0.8≦y≦1.2, with Mcontaining a 3d transition metal, where the Li_(x)M_(y)PO₄ encompassesthat with the grain size not larger than 10 μm.

The positive electrode active material according to the presentinvention contains Li_(x)M_(y)PO₄ with the grain size not larger than 10μm. In this manner, the positive electrode active material is of a grainsize distribution enabling e.g., lithium, as charge carrier, to bediffused sufficiently in the grains of the positive electrode activematerial.

The present invention also provides a positive electrode active materialcontaining a compound represented by the general formulaLi_(x)(Fe_(y)M_(1−y))PO₄, where 0.9≦x≦1.1 and 0<y≦1, with M containing a3d transition metal, wherein, in a spectrum for theLi_(x)(Fe_(y)M_(1−y))PO₄ obtained by the Moessbauer spectroscopicmethod, A/B is less than 0.3, where A is the area strength of a spectrumobtained by the Moessbauer spectroscopic method of not less than 0.1mm/sec and not larger than 0.7 mm/sec and B is the area strength of aspectrum obtained by the Moessbauer spectroscopic method not less than0.8 mm/sec and not larger than 1.5 mm/sec.

With this positive electrode active material, according to the presentinvention, since A/B is less than 0.3, the quantity of electrochemicallyinert impurities is small, thus realizing a high capacity.

The present invention also provides a non-aqueous electrolyte secondarybattery including a positive electrode having a positive electrodeactive material containing a compound represented by the general formulaLi_(x)M_(y)PO₄, where 0<x≦2 and 0.8≦y≦1.2, with M containing a 3dtransition metal, a negative electrode having a negative electrodeactive material, the positive electrode active material and the negativeelectrode active material being capable of reversibly doping/undopinglithium, and a non-aqueous electrolyte, wherein the Li_(x)M_(y)PO₄encompasses that with the grain size not larger than 10 μm.

The non-aqueous electrolyte secondary battery according to the presentinvention contains Li_(x)M_(y)PO₄, with the grain size not larger than10 μm, as a positive electrode active material. This positive electrodeactive material is of such a grain size distribution that enableslithium as a charge carrier to be diffused sufficiently in the grains.Thus, the non-aqueous electrolyte secondary battery is of high capacity.

The present invention also provides a non-aqueous electrolyte secondarybattery including a positive electrode having a positive electrodeactive material containing a compound represented by the general formulaLi_(x)(Fe_(y)M_(1−y))PO₄, where 0.9≦x≦1.1 and 0<y≦1, with M containing a3d transition metal, a negative electrode having a negative electrodeactive material, the positive electrode active material and the negativeelectrode active material being capable of reversibly doping/undopinglithium, and a non-aqueous electrolyte, wherein, in a spectrum for theLi_(x)(Fe_(y)M_(1−y))PO₄ obtained by the Moessbauer spectroscopicmethod, A/B is less than 0.3, where A is the area strength of a spectrumobtained by the Moessbauer spectroscopic method not less than 0.1 mm/secand not larger than 0.7 mm/sec and B is the area strength of a spectrumobtained by the Moessbauer spectroscopic method not less than 0.8 mm/secand not larger than 1.5 mm/sec.

The non-aqueous electrolyte secondary battery, according to the presentinvention, is of the value of A/B less than 0.3, and contains thepositive electrode active material with low content of electrochemicallyinert impurities, thus realizing a non-aqueous electrolyte secondarybattery of a high capacity.

It is another object of the present invention to provide a method forproducing a positive electrode active material which, if used in abattery, realizes a high battery capacity.

For accomplishing the above object, the present invention provides amethod for producing a positive electrode active material including amixing step of mixing a starting material for synthesis of a compoundrepresented by the general formula Li_(x)M_(y)PO₄, where 0<x≦2 and0.8≦y≦1.2, with M containing a 3d transition metal, and a sintering stepof sintering and reacting the precursor obtained in the mixing step,wherein, in the sintering step, the precursor is sintered at atemperature not lower than 400° C. and not higher than 700° C.

In the manufacturing method for the positive electrode active materialaccording to the present invention, the precursor of Li_(x)M_(y)PO₄ issintered in the sintering step at a temperature not lower than 400° C.and not higher than 700° C. So, the chemical reaction andcrystallization proceed uniformly, without the crystallizationproceeding excessively, to yield impurity-free single-phaseLi_(x)M_(y)PO₄. Also, the powder characteristics of Li_(x)M_(y)PO₄ arechanged dramatically due to the difference in the temperature ofsintering the precursor of Li_(x)M_(y)PO₄.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an illustrative structure of anon-aqueous electrolyte secondary battery embodying the presentinvention.

FIG. 2 is a graph showing a powder X-ray diffraction pattern of LiFePO₄synthesized in samples 1 to 5.

FIG. 3 is a graph showing the relation between the sintering temperatureof LiFePO₄ synthesized in samples 1 to 5 and the charging/dischargingcapacity of the battery.

FIG. 4 is a graph showing the relation between the sintering temperatureof LiFePO₄ synthesized in samples 1 to 5 and the volumetric grain sizedistribution of the battery.

FIG. 5 is a graph showing the relation between the sintering temperatureof LiFePO₄ synthesized in samples 1 to 5 and the volumetric grain sizedistribution of the battery.

FIG. 6 is a graph showing the relation between the sintering temperatureof LiFePO₄ synthesized in samples 1 to 5 and the volumetric cumulativediameter of the battery.

FIG. 7 is a photo, taken by a scanning microscope, for showing the grainshape of LiFePO₄ sintered at 500° C.

FIG. 8 is a photo, taken by a scanning microscope, for showing the grainshape of LiFePO₄ sintered at 600° C.

FIG. 9 is a photo, taken by a scanning microscope, for showing the grainshape of LiFePO₄ sintered at 700° C.

FIG. 10 is a graph showing BET specific surface area of LiFePO₄synthesized in samples 1 to 5.

FIG. 11 is a graph showing a powder X-ray diffraction pattern of LiFePO₄synthesized in samples 1, 5 and 6.

FIG. 12 is a graph showing charging/discharging characteristics of abattery prepared in sample 1.

FIG. 13 is a graph showing cyclic characteristics of a battery preparedin sample 1.

FIG. 14 is a graph showing charging/discharging characteristics of abattery prepared in sample 5.

FIG. 15 is a graph showing charging/discharging characteristics of abattery prepared in sample 6.

FIG. 16 is a graph showing an X-ray diffraction pattern ofLi(Mn_(0.6)Fe_(0.4))PO₄.

FIG. 17 shows charging/discharging characteristics of a battery preparedfrom Li(Mn_(0.6)Fe_(0.4))PO₄.

FIG. 18 shows grain size distribution of Li(Mn_(0.6)Fe_(0.4))PO₄obtained on sintering at 600° C.

FIG. 19 is a Moessbauer spectrum diagram of LiFePO₄ of sample 6synthesized at a sintering temperature of 320° C.

FIG. 20 is a Moessbauer spectrum diagram of LiFePO₄ of sample 2synthesized at a sintering temperature of 400° C.

FIG. 21 is a Moessbauer spectrum diagram of LiFePO₄ of sample 6synthesized at a sintering temperature of 600° C.

FIG. 22 is a Moessbauer spectrum diagram of Fe²⁺ of LiFePO₄ of sample 6.

FIG. 23 is a Moessbauer spectrum diagram of Fe³⁺ of LiFePO₄ of sample 6.

FIG. 24 is a Moessbauer spectrum diagram of Fe²⁺ of LiFePO₄ of sample 2.

FIG. 25 is a Moessbauer spectrum diagram of Fe³⁺ of LiFePO₄ of sample 2.

FIG. 26 is a Moessbauer spectrum diagram of Fe²⁺ of LiFePO₄ of sample 1.

FIG. 27 is a Moessbauer spectrum diagram of Fe₃ ⁺ of LiFePO₄ of sample1.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the drawings, the present invention will be explained indetail.

Referring first to FIG. 1, a non-aqueous electrolyte battery 1 accordingto the present invention includes a negative electrode 2, a negativeelectrode can 3, accommodating a negative electrode 4, a positiveelectrode 4, a positive electrode can 5, a separator 6 and an insulatinggasket 7. The non-aqueous electrolyte is charged into the negativeelectrode can 3 and the positive electrode can 5.

The negative electrode 2 is comprised of a negative electrode currentcollector on which is deposited a layer of a negative electrode activematerial. A nickel foil, for example, is used as a negative electrodecurrent collector.

As the negative electrode active materia, such a material that iscapable of doping/undoping lithium, is used. For example, metal lithium,lithium alloys, an electrically conductive high polymer material dopedwith lithium, and a laminated compound, such as a carbon material or ametal oxide, are used.

As a binder contained in the layer of the negative electrode activematerial, any suitable known binders routinely used as a binder for thelayer of the negative electrode active material for this sort of thenon-aqueous electrolyte secondary battery may be used.

As the negative electrode 2, a metal lithium foil, operating as anegative electrode active material, may be used.

The negative electrode can 3 is used for accommodating the negativeelectrode 2 and also operates as an external negative electrode of thenon-aqueous electrolyte battery 1.

On the positive electrode current collector of the positive electrode 4,there is formed a layer of a positive electrode active materialcontaining a positive electrode active material.

This positive electrode active material contains a compound having anolivinic structure and which is represented by the general formulaLi_(x)M_(y)PO₄, where x is such that 0<x≦2, y is such that 0.8≦y≦1.2 andM contains at least one of 3d transition metals. The manufacturingmethod for the positive electrode active material will be explainedsubsequently.

The compounds represented by the general formula Li_(x)M_(y)PO₄ may beenumerated by, for example, Li_(x)Fe_(y)PO₄, Li_(x)Mn_(y)PO₄,Li_(x)Co_(y)PO₄, Li_(x)Ni_(y)PO₄, Li_(x)Cu_(y)PO₄, Li_(x)(Fe,Mn)_(y)PO₄, Li_(x)(Fe, CO)_(y)PO₄, Li_(x)(Fe, Ni)_(y)PO₄, Li_(x)(Cu,Mn)_(y)PO₄, Li_(x)(Cu, CO)_(y)PO₄, Li_(x)(Cu, Ni)_(y)PO₄, Li_(x)(Mn,Ti)_(y)PO₄, Li_(x)(Mn, Zn)_(y)PO₄ and Li_(x)(Mn, Mg)_(y)PO₄, where theproportions of elements in parentheses ( ) are arbitrary.

This Li_(x)M_(y)PO₄ includes that with the grain size not larger than 10μm. If, as Li_(x)M_(y)PO₄ contained by the positive electrode activematerial, Li_(x)M_(y)PO₄, with the grain size not larger than 10 μm, isnot contained, the grain size distribution is not optimum, so thatlithium as a charge carrier cannot migrate sufficiently in the grain ofthe positive electrode active material.

The 10% cumulative volumetric diameter of Li_(x)M_(y)PO₄ preferably isnot less than 1 μm. If the 10% cumulative volume diameter is larger than1 μm, it may be feared that coarse grained LiM_(y)PO₄, produced due toexcess progress of crystallization, accounts for the major portion ofLiM_(y)PO₄, such that lithium as charge carrier cannot be diffusedsmoothly in the grain of the positive electrode active material.

Moreover, Li_(x)M_(y)PO₄ preferably has the Brunauer Emmett Teller (BET)specific surface area not lower than 0.5 m²/g. With a positive electrodemixture of a larger grain size, the specific surface area becomessmaller. If the large current is allowed to flow under such conditions,that is if a large quantity of lithium ions are introduced in a shorttime into the active material, the diffusion of lithium in the activematerial cannot catch up with the lithium supply from outside, with theresult that the apparent capacity is decreased. So, if desired toprocure sufficient capacity under a large current, technical measuresare required to increase the specific surface area, and also to reducethe grain size, as described above.

By increasing the BET specific surface area of Li_(x)M_(y)PO₄ to notless than 0.5 m²/g, it is possible to promote lithium diffusion in theactive material to secure a sufficient capacity even under a largecurrent.

As for a compound represented by the general formula Li_(x)M_(y)PO₄where M contains Fe as a 3d transition metal, that is a compoundLi_(x)(Fe_(y)M_(1−y))PO₄, where x is such that 0.9≦x≦1.1 and y is suchthat 0<y≦1, with M being a 3d transition metal, such a compound in whichA/B is less than 0.3, is used, in which, in the spectrum obtained by theMoessbauer spectroscopic method, A is the area strength of the spectrumof an isomer shift value not less than 0.1 mm/sec and not larger than0.7 mm/sec and B the area strength of the spectrum of an isomeric shiftvalue not less than 0.8 mm/sec and not larger than 1.5 mm/sec.

For example, in LiFePO₄, that is Li_(x)(Fe_(y)M_(1−y))PO₄ where x is 1and y is 0, Moessbauer spectroscopic measurement reveals a doublet, inwhich, as the Moessbauer spectrum corresponding to Fe²⁺, the isomericshift value is approximately 1.2 mm/sec and the quadrupolar fission ofapproximately 2.9 mm/sec. Also, if Fe²⁺ is oxidized such that Fe³⁺exists in LiFePO₄, such a doublet in which the isomeric shift value isnot less than 0.1 mm/sec and not larger than 0.7 mm/sec is observed asthe Moessbauer spectrum corresponding to Fe³⁺.

During initial charging process, LiFePO₄ is freed of Li, while Fe²⁺ isoxidized to Fe³⁺. If, in the pre-initial-charging state, Fe²⁺ iscontained in LiFePO₄, the number of electrons contributing to batteryreaction is diminished, so that the charging capacity in the lithium ionsecondary battery is lowered.

Since the lithium ion secondary battery uses an Li-free material, suchas carbon, as the negative electrode, the initial charging capacitydetermines the subsequent battery capacity. On the other hand, if, inthe lithium ion secondary battery, the Li-containing material is used asthe negative electrode, but the Fe³⁺-containing phase iselectro-chemically inert, the battery capacity tends to be lowered dueto this inert phase. Thus, in the pre-initial-charging state, Fe³⁺present in LiFePO₄ is desirably as small as possible.

The above-mentioned area strength A is proportionate to the amount ofFe³⁺ present in LiFePO₄, whilst the area strength B is proportionate tothe amount of Fe³⁺ present in LiFePO₄. Therefore, in LiFePO₄ in whichA/B is less than 0.3, the amount of Fe³⁺ is small, such that a highcapacity is achieved in the case of the non-aqueous electrolytesecondary battery containing this LiFePO₄ as the positive electrodeactive material.

The positive electrode current collector may, for example, be analuminum foil.

As a binder contained in the positive electrode active material, anysuitable known resin material, routinely used as a binder for a layer ofthe positive electrode active material of this sort of the non-aqueouselectrolyte battery, may be used.

The positive electrode can 5 accommodates the positive electrode 4 andserves as an external positive electrode of the non-aqueous electrolytebattery 1.

The separator 6, used for separating the positive electrode 4 from thenegative electrode 2, may be formed of a known material routinely used aseparator of this sort of the non-aqueous electrolyte battery, and may,for example, be a high molecular film, such as a polypropylene film.From the relation between lithium ion conductivity and the energydensity, the separator needs to be as thin as possible. Specifically,the separator thickness of, for example, not larger than 50 μm, isdesirable.

The insulating gasket 7, built and unified into the negative electrodecan 3, is used for preventing leakage of the non-aqueous electrolytecharged into the negative electrode can 3 and the positive electrode can5.

As the non-aqueous electrolyte, a solution obtained on dissolvingan-electrolyte in a non-protonic non-aqueous solvent is used.

The non-aqueous solvent may be exemplified by, for example, propylenecarbonate, ethylene carbonate, butylene carbonate, vinylene carbonate,γ-butyrolactone, sulforane, 1,2-dimethoxyethane, 1,2-diethoxyethane,2-methyltetrahydrofuran, 3-methyl 1,3-dioxorane, methyl propionate,methyl lactate, dimethyl carbonate, diethyl carbonate and dipropylcarbonate. Especially, from voltage stability, cyclic carbonates, suchas propylene carbonate or vinylene carbonate, or chain carbonates, suchas dimethyl carbonate, diethyl carbonate or dipropyl carbonate, arepreferably used. As this non-aqueous solvent, only one type non-aqueoussolvent or a mixture of two or more non-aqueous solvents may be used.

As the electrolyte, dissolved in the non-aqueous solvent, lithium salts,such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃ or LiN(CF₃SO₂)₂, may beused. Of these lithium salts, LiPF₆ or LiBF₄ may preferably be used.

The non-aqueous electrolyte secondary battery 1, employing theabove-mentioned Li_(x)M_(y)PO₄ as the positive electrode activematerial, is manufactured, e.g., by the following method:

For preparing the negative electrode 2, a negative electrode activematerial and a binder are dispersed in a solvent to prepare a slurriednegative electrode mixture. The so-produced negative electrode mixtureis evenly coated on a current collector and dried in situ to form alayer of the negative electrode active material to complete the negativeelectrode 2. As the binder for the negative electrode mixture, anysuitable known binder may be used. Alternatively, the negative electrodemixture may be added to with any suitable known additives. On the otherhand, metal lithium, as a negative electrode active material, may bedirectly used as the negative electrode 2.

For preparing the positive electrode 4, Li_(x)M_(y)PO₄, which proves thepositive electrode active material, and a binder, are dispersed in asolvent to prepare a slurried positive electrode mixture. The positiveelectrode mixture, thus produced, is evenly coated on the currentcollector and dried in situ to form a layer of the positive electrodeactive material to complete the positive electrode 4. As the binder ofthe positive electrode mixture, any suitable known binder may be used.Alternatively, known additives may be added to the positive electrodemixture.

The non-aqueous electrolyte is prepared by dissolving an electrolytesalt in a non-aqueous solvent.

The negative electrode is accommodated in the negative electrode can 3,while the positive electrode is accommodated in the positive electrodecan 5. The separator 6 in the form of a polypropylene porous film isarranged between the negative electrode and the positive electrode 4.The non-aqueous electrolyte is charged into the negative electrode can 3and the positive electrode can 5. These cans 3, 5 are caulked togetherand fastened to each other to complete the non-aqueous electrolytebattery 1.

Meanwhile, in the manufacturing method for a positive electrode activematerial according to the present invention, a compound having anolivinic structure and which is represented by the general formulaLi_(x)M_(y)PO₄, where x is such that 0.9≦x≦1.1 and y is such that 0<y≦1,with M containing a 3d transition metal, such as LiFePO₄, is synthesizedby the following method:

First, as a starting material for synthesis, iron acetate (Fe(CH₃COO)₂),ammonium hydrogen phosphate (NH₄H₂PO₄) and lithium carbonate (Li₂CO₃)were mixed together at a pre-set ratio to give a precursor. The startingmaterials for synthesis need to be mixed thoroughly. By mixing thestarting materials for synthesis sufficiently, the starting materialsare mixed evenly to render it possible to synthesize LiFePO₄ at a lowertemperature than conventionally.

This precursor then is sintered at a pre-set temperature in anatmosphere of an inert gas, such as nitrogen, to synthesize LiFePO₄.

Heretofore, LiFePO₄ was sintered at a higher temperature of, forexample, 800° C. If the sintering temperature is high, the energyconsumption is correspondingly increased, whilst the load applied to thereaction apparatus is higher.

Thus, by sufficiently mixing the starting materials for synthesis togive a precursor, and by sintering the precursor in a nitrogen stream,it has become possible to synthesize LiFePO₄ at a temperature markedlylower than 800° C. so far used. That is, LiFePO₄ can now be synthesizedat a temperature markedly lower than 800° C. heretofore used to providefor wider latitude of selection of the temperature with which to sinterthe precursor (referred to below as sintering temperature). The presentinventors have directed attention to the relation between the sinteringtemperature with which to sinter the precursor and the capacity of thebattery employing LiFePO₄ as an active material to search into theoptimum sintering temperature for LiFePO₄.

As a result of this search, the sintering temperature of LiFePO₄ is setto not less than 400° C. and not higher than 700° C. The sinteringtemperature of LiFePO₄ is preferably not less than 400° C. and nothigher than 600° C.

If the sintering temperature of LiFePO₄ is lower than 400° C., therepersists a phase containing e.g., trivalent iron compounds, asimpurities, that is Fe³⁺, such that homogeneous LiFePO₄ cannot beproduced. If the sintering temperature of LiFePO₄ is higher than 700°C., crystallization proceeds excessively, such that there is a risk thatit becomes difficult to suppress the precipitation of impurities.

Meanwhile, in the above-described manufacturing method for the positiveelectrode active material, the precursor is preferably de-aerated, priorto its sintering, to remove air contained in the precursor.

If air is left in the precursor, Fe²⁺ in iron acetate, as a bivalentiron compound, is oxidized by oxygen in air and turned into Fe³⁺ duringsintering of LiFePO₄. The result is that the trivalent iron compound, asan impurity, is mixed into the Product LiFePO₄. By removing aircontained in the precursor by de-aerating processing, it is possible toprevent oxidation of Fe²⁺ in iron acetate. The result is that notrivalent iron compound is mixed into the produce LiFePO₄ to render itpossible to prepare single-phase LiFePO₄.

As the starting materials for synthesis of LiFePO₄, a variety ofstarting materials, such as lithium hydroxide, lithium nitrate, lithiumacetate, lithium phosphate, iron phosphate (II) or iron oxide (II), maybe used in addition to the above-mentioned compounds. For sintering athigher temperatures of not lower than 400° C. and not higher than 700°C., it is desirable to use a starting materials of higher reactivity.

The non-aqueous electrolyte secondary battery 1, prepared as describedabove, contains Li_(x)M_(y)PO₄ as the positive electrode activematerial.

This positive electrode active material, containing Li_(x)M_(y)PO₄,having the grain size not larger than 10 μm, exhibits grain sizedistribution optimum for sufficient diffusion of lithium as chargecarrier to occur sufficiently. So, with the non-aqueous electrolytebattery 1, lithium doping/undoping occurs satisfactorily, thus realizingsuperior cyclic characteristics and high capacity.

Moreover, with the present positive electrode active material,containing Li_(x)M_(y)PO₄, having the 10% volume cumulative diameter notlarger than 1 μm, is of a grain size distribution more suited fordiffusion of lithium as a charge carrier to occur more smoothly. So, thenon-aqueous electrolyte battery 1, in which doping/undoping of lithiumoccurs satisfactorily, exhibits superior cyclic characteristics and highcapacity.

In the above-described manufacturing method for the positive electrodeactive material, the starting materials for synthesis of a compoundhaving the general formula Li_(x)M_(y)PO₄, for example, LiFePO₄, aremixed together to form a precursor, which precursor is then sintered ata temperature not lower than 400° C. and not higher than 700° C., sothat the chemical reaction and crystallization proceed evenly whilstcrystallization does not proceed excessively. This gives impurity-freesingle-phase LiFePO₄ as a positive electrode active material. Thus, thepositive electrode active material is able to achieve a high capacityexceeding 120 mAh/g of a conventional non-aqueous electrolyte battery.

Moreover, by setting the sintering temperature range to not lower than400° C. and not higher than 600° C., it is possible to realize a realcapacity approaching to 170 mAh/g which is the theoretical capacity ofLiFePO₄.

It is noted that the positive electrode active material of the presentinvention is not limited to LiFePO₄ as described above, but may also beapplied to any suitable compound represented by the general formulaLi_(x)M_(y)PO₄.

Moreover, the present invention is not limited to this and may beapplied to the use as the non-aqueous electrolyte of a solid electrolyteor a gelated solid electrolyte containing a swelling solvent. Thepresent invention may also be applied to a variety of shapes of thenon-aqueous electrolyte secondary batteries, such as a cylindricalshape, a square shape, a coin or a button shape, or to a variety ofsizes of the non-aqueous electrolyte secondary battery, such as a thintype or large-sized batteries.

Although the foregoing description has been made of a manufacturingmethod for a positive electrode active material including mixing andsintering powders of compounds as starting materials for synthesis ofLiFePO₄. The present invention is, however, not limited to this methodsince it may be applied to the solid-phase reaction or to a variety ofreactions other than the solid phase reaction to synthesize a compoundrepresented by the general formula Li_(x)M_(y)PO₄.

The present invention will hereinafter be explained with reference tospecified Examples and Comparative Examples based on experimentalresults.

<Experiment 1>

In Experiment 1, a compound represented by the general formulaLi_(x)M_(y)PO₄ was prepared as a positive electrode active material andnon-aqueous electrolyte secondary batteries employing this positiveelectrode active material were prepared as test cells to evaluatevarious characteristics thereof.

First, in order to valuate the difference in characteristics ofnon-aqueous electrolyte secondary batteries caused by the difference inthe grain size distribution of the positive electrode active material,positive electrode active materials were prepared using variablesintering temperatures, and test cells were prepared using thesepositive electrode active materials.

Sample 1

First, LiFePO₄ was prepared as a positive electrode active material withthe sintering temperature of 600° C.

For preparing LiFePO₄, ammonium dihydrogen phosphate (NH₄H₂PO₄) as astarting materials of a coarser crystallite size was sufficientlypulverized at the outset. Then, iron acetate (Fe(CH₃COO)₂), ammoniumdihydrogen phosphate (NH₄H₂PO₄) and lithium carbonate (Li₂CO₃) weremixed sufficiently to a molar ratio of 2:2:1 to give a precursor.

The precursor was then calcined at 300° C. for 12 hours and subsequentlysintered in a nitrogen atmosphere for 24 hours to synthesize LiFePO₄.

A battery was prepared using LiFePO₄, thus prepared, as a positiveelectrode active material.

70 wt % of dried LiFePO₄, as the positive electrode active material, 25wt % of acetylene black, as an electrically conductive material, and 5wt % of polyvinylidene fluoride, as a binder, were evenly mixed intodimethyl formamide as a solvent to prepare a paste-like positiveelectrode mixture. Meanwhile, #1300 manufactured by Aldrich Inc. wasused as the polyvinylidene fluoride.

This positive electrode mixture was applied to an aluminum mesh, as acurrent collector, and dried in situ in a dry argon atmosphere at 100°C. for one hour to form a layer of the positive electrode activematerial.

The aluminum mesh, on which the layer of the positive electrode activematerial was formed, was punched to a disc 15 mm in diameter to form apellet-like positive electrode. Meanwhile, this positive electrodecarries 60 mg of the active material.

A metal lithium foil was punched to substantially the same shape as thepositive electrode and used as a negative electrode.

In a mixed solvent of equal parts in volume of propylene carbonate anddimethyl carbonate was dissolved LiPF₆ at a concentration of 1 mol/l toprepare a non-aqueous electrolytic solution.

The positive electrode, prepared as described above, was accommodated inthe positive electrode can, whilst the negative electrode wasaccommodated in the negative electrode can and the separator wasarranged between the positive electrode and the negative electrode. Thenon-aqueous electrolytic solution was charged into the positiveelectrode can and the negative electrode can. The electrode cans 3, 5are caulked fixedly through the insulating gasket 7 to complete a 2025type coin-shaped test cell.

Sample 2

LiFePO₄ was prepared in the same way as in Sample 1, except using thesintering temperature of 400° C., and a test cell was prepared usingthis positive electrode active material.

Sample 3

LiFePO₄ was prepared in the same way as in Sample 1, except using thesintering temperature of 500° C., and a test cell was prepared usingthis positive electrode active material.

Sample 4

LiFePO₄ was prepared in the same way as in Sample 1, except using thesintering temperature of 700° C., and a test cell was prepared usingthis positive electrode active material.

Sample 5

LiFePO₄ was prepared in the same way as in Sample 1, except using thesintering temperature of 800° C., and a test cell was prepared usingthis positive electrode active material.

Then, measurement was made of the powder X-ray diffraction pattern ofthe LiFePO₄, as a positive electrode active material, prepared by theabove-described method. The measurement conditions of the powder X-raydiffraction were as follows:

-   apparatus used: RIGAKU RINT 2500 rotary counter pair negative    electrode-   goniometer: vertical type standard, radius 185 mm-   counter monochromator: used-   filter: not used-   slit width    -   divergent slit (DS)=1°    -   receiving slit (RS)=1°    -   scattering slit (SS)=0.15 mm-   counter device: scintillation counter-   measurement method: reflection method, continuous scan-   scanning range: 2θ=10° to 80°-   scanning speed: 4°/minute

The powder X-ray diffraction pattern of LiFePO₄, synthesized in Example1, is shown in FIG. 2, from which it is seen that a single-phase LiFePO₄has been obtained since the presence of the impurity other than LiFePO₄is not confirmed in the product.

The test cells, prepared as samples 1 to 4, were subjected to thecharging/discharging test, in which each test cell was charged byconstant current charging and, when the battery voltage reached 4.5V,the charging system was switched from the constant current charging toconstant voltage charging, and charging was carried out as the voltageof 4.5 V was kept. The charging was stopped when the current fell below0.01 mA/cm². The discharging then was carried out and stopped at a timepoint when the battery voltage was lowered to 2.0 V. Meanwhile,charging/discharging was carried out at ambient temperature (23° C.),with the current density at this time being 0.12 mA/cm².

The relation between the sintering temperature of LiFePO₄, synthesizedin Samples 1 to 5 and the battery charging/discharging capacity, as theresult of the charging/discharging test, is shown in FIG. 3, from whichit is seen that the non-aqueous electrolyte secondary battery comes tohave a high capacity by sintering LiFePO₄ as the positive electrodeactive material at a temperature not lower than 400° C. and not higherthan 700° C. It has also been seen that, when the sintering temperatureof the precursor is not lower than 400° C. and not higher than 600° C.,the non-aqueous electrolyte secondary battery comes to have an extremelyhigh capacity.

Of the positive electrode active materials, synthesized as samples 1 to5, measurements were made of the volumetric grain size distribution. Formeasuring the volumetric grain size distribution, a volume grain sizedistribution measurement device, manufactured by HORIBA SEISAKUSHO CO.LTD. under the trade name of Micro-Lack grain size analyzer LA-920, wasused. Using this measurement device, the scattering of the laser lightwas measured to measure the volumetric grain size distribution. Themeasured results of the volumetric grain size distribution are shown inFIG. 4.

As may be seen from FIG. 4, if the sintering temperature is higher than600° C., the volumetric distribution of LiFePO₄ with the grain sizelarger than 10 μm, is increased as the center of distribution is shiftedtowards the coarse grain side. On the other hand, the volumetricdistribution of LiFePO₄ with the grain size not larger than 10 μm isdecreased appreciably.

If the sintering temperature is not higher than 600° C., the volumetricdistribution of LiFePO₄, having the grain size not larger than 10 μm, isincreased as the center of distribution is shifted towards the finergrain side.

From the results of the volumetric grain size, shown in FIG. 4, and fromthe results between the sintering temperature shown in FIG. 3 and thebattery charging/discharging capacity, it has been seen that it is theLiFePO₄ grains not larger 10 μm that are contributing to the batterycapacity.

From this it is seen that the non-aqueous electrolyte secondary batterycontaining LiFePO₄ having a grain size not larger than 10 μm as thepositive electrode active material comes to have an extremely highcapacity.

The relation between the sintering temperature and the cumulativevolumetric grain size of LiFePO₄, as found from the measured results ofthe volumetric grain size distribution, is shown in FIG. 5, from whichit is seen that there is a definite correlation between the grain sizeof LiFePO₄ and the sintering temperature of LiFePO₄. FIG. 6 shows thesame relation as that shown in FIG. 5, but with the range of 0.1 to 10μm of the grain size increased in scale.

It is seen from FIG. 6 that, if the sintering temperature of LiFePO₄ isnot higher than 600° C., LiFePO₄, having a grain not larger than 1 μm,accounts for not less than 10%. On the other hand, if the sinteringtemperature of LiFePO₄ is higher than 600° C., LiFePO₄ with the grainsize not larger than 1 μm is less than 10%.

From the results of the relation between the sintering temperature andthe cumulative volumetric grain size (for the grain size ranging between0.1 and 10 μm) of LiFePO₄, shown in FIG. 6, and from the results of therelation between the sintering temperature and the batterycharging/discharging capacity, the non-aqueous electrolyte secondarybattery preferably contains LiFePO₄, having the 10% volumetriccumulative grain size not larger than 1 μm, as a positive electrodeactive material, whereby the battery comes ro have a high real capacityapproaching to the theoretical capacity of LiFePO₄.

The positive electrode active materials of the samples 3, 1 and 4, withthe LiFePO₄ sintering temperatures of 500° C., 600° C. and 700° C.,respectively, were observed over a scanning microscope. The respectivemicroscopic photos are shown in FIGS. 7, 8 and 9, from which it may beclearly seen that LiFePO₄ undergoes specific growth with rise in thesintering temperature to prove coarse sized grains. This is insatisfactory agreement with the results of the volumetric grain sizedistribution shown in FIG. 5. From this it is seen that crystallizationof LiFePO₄ proceeds with rise in the sintering temperature.

The BET specific surface area was also measured of LiFePO₄ synthesizedin samples 1 to 5. The measured results of the BET specific surface areaare shown in FIG. 10, in which there are also plotted measured resultson LiFePO₄, in which the sintering temperature is changed more finely,in addition to those on the samples 1 to 5.

It is seen from FIG. 10 that the BET specific surface area is changedmonotonously with rise in the sintering temperature of LiFePO₄, with thechange width being of an extremely large value ranging from not lessthan 20 m²/g to 0.5 m²/g.

It is seen from comparison of FIG. 10 to FIG. 3 showing the sinteringtemperature and the discharging capacity of LiFePO₄ that a real capacityalmost as high as the theoretical capacity of LiFePO₄ is achieved whenthe BET specific surface area of LiFePO₄ as the positive electrodeactive material is not less than 0.5 m²/g and more preferably is notless than 2 m²/g.

For scrutinizing into an optimum sintering temperature of the positiveelectrode active material, a positive electrode active material wassynthesized at a sintering temperature lower than that usedconventionally and, using the positive electrode active material, a testcell was prepared as sample 6.

Sample 6

LiFePO₄ was prepared in the same way as in sample 1 except using thesintering temperature of 320° C. and a test cell was prepared using theso-produced LiFePO₄ as a positive electrode active material.

First, the powder X-ray diffraction pattern was measured of the positiveelectrode active material synthesized in sample 6 and positive electrodeactive material synthesized in samples 1 and 5, that is LiFePO₄. Themeasured results are shown in FIG. 11, from which it is seen that, inLiFePO₄ synthesized in samples 1, 5 and 6, no impurities other thanLiFePO₄ are confirmed to be present in the product such thatsingle-phase LiFePO₄ has been produced in each sample.

A charging/discharging test was then conducted on the test cellsprepared in samples 1, 5 and 6.

The charging/discharging characteristics of the sample 1 are shown inFIG. 12, from which it is seen that the battery of sample 1 employingLiFePO₄ obtained on sintering the precursor at 600° C. as a positiveelectrode active material shows a flat potential in the vicinity of 3.4V. Moreover, in this battery, a reversible charging/discharging capacityof 163 mAh/g is produced in this battery. This value of 163 mAh/gapproaches 170 mAh/g which is the theoretical capacity of LiFePO₄.

The relation between the number of cycles and the charging/dischargingcapacity of the battery of sample 1 is shown in FIG. 13, from which itis seen that cyclic deterioration of the charging/discharging capacityis as low as 0.1%/cycle thus testifying to stable batterycharacteristics.

On the other hand, FIG. 14 shows that the charging/discharging capacityof the sample 5 battery is extremely low. This is presumably ascribableto the fact that, since the sintering temperature of LiFePO₄ is as highas 800° C. so that crystallization proceeds excessively to prohibitsufficient lithium diffusion in the LiFePO₄ particles.

It is also seen that, with the battery of sample 5, thecharging/discharging capacity achieved is extremely low, as shown inFIG. 14. This is probably due to the fact that the sintering temperatureof LiFePO₄ is as high ss 800° C. such that crystallization proceedsexcessively such that lithium diffusion does not occur sufficiently inthe LiFePO₄ grains.

It is seen from the above results that a high capacity is achieved withLiFePO₄, as a positive electrode active material, obtained with thesintering temperature of not less than 400° C. and not higher than 700°C.

Moreover, it is seen that a high real capacity exceeding 120 mAh/g ofthe conventional non-aqueous electrolyte secondary battery can beachieved by adding MnCO₃ into the starting materials and by sinteringLiFePO₄ at the sintering temperature of not less than 400° C. and nothigher than 700° C.

Moreover, Li(Mn_(0.6)Fe_(0.4))PO₄ was prepared by adding MnCO₃ into thestarting materials and by sintering in a similar manner. The x-raydiffraction diagram of the produced Li(Mn_(0.6)Fe_(0.4))PO₄ is shown inFIG. 16, from which it is seen that Li(Mn_(0.6)Fe_(0.4))PO₄ is free ofimpurities and is of the single-phase olivinic structure.

The charging/discharging characteristics of a battery prepared in asimilar manner using Li(Mn_(0.6)Fe_(0.4))PO₄ obtained on sintering at600° C. are shown in FIG. 17, from which it is seen that not only thecapacity as high as 150 mA/h/g is realized but also a capacity near 4Vis newly observed, thereby improving the energy density.

The measured results of the grain size distribution ofLi(M_(0.6)Fe_(0.4))PO₄ obtained on sintering at 600° C. are shown inFIG. 17, from which it is seen that this Li(Mn_(0.6)Fe_(0.4))PO₄contains Li(Mn_(0.6)Fe_(0.4))PO₄ with the grain size not larger than 10μm, with the 10% cumulative volumetric grain size being within a rangeof not larger than 1 μm.

<Experiment 2>

In Experiment 2, measurement was made of the Moessbauer spectrum ofLiFePO₄ of the sample 6, sample 2 and the sample 1, containing Feresponsible for the observed Moessbauer effect, and obtained at thesintering temperatures of 320° C., 400° C. and 600° C., respectively,amongst the positive electrode active materials prepared in Experiment1, using the Moessbauer spectroscopic method.

In measuring the Moessbauer spectrum, 50 mg of LuFePO₄, as sample, wascharged in plural holes in a lead plate 0.5 mm in thickness and 15 mm indiameter, and both sides of the holes were sealed with a tape, and ⁵⁷Coof 1.85 Gbq was illuminated on the plate charged with the sample.

The measured results of the spectrum of LiFePO₄, as samples 6, 2 and 1,obtained by Moessbauer spectroscopic method, are shown in FIGS. 19, 20and 21, respectively.

The spectra of Fe²⁺ and Fe³⁺, obtained on fitting the Moessbauerspectrum of LiFePO₄ of sample 6 shown in FIG. 19, are shown in FIGS. 22and 23, respectively.

The spectra of Fe²⁺ and Fe3T, obtained on fitting the Moessbauerspectrum of LiFePO₄ of sample 2 shown in FIG. 20, are shown in FIGS. 24and 25, respectively.

The spectra of Fe²⁺ and Fe3T, obtained on fitting the Moessbauerspectrum of LiFePO₄ of sample 2 shown in FIG. 21, are shown in FIGS. 26and 27, respectively.

The spectrum inherent to LiFePO₄ is a doublet with an isomeric shiftcorresponding to Fe²⁺ being approximately 1.2 mm/sec and quadrupolarfission being approximately 2.9 mm/sec, as shown in FIGS. 22, 24 and 26.

On the other hand, with LiFePO₄, as sample 6, with the sinteringtemperature of 320° C., a broad doublet with an isomeric shiftcorresponding to Fe³⁺ of approximately 0.4 mm/sec and with a quadrupolarfission of approximately 0.8 mm/sec, as shown in FIG. 23.

The value of A/B, where A is the area strength of the doubletcorresponding to Fe³⁺, that is the area strength of the spectrum withthe isomeric shift not less than 0.1 mm/sec and not larger than 0.7mm/sec and B is the area strength of the doublet corresponding to Fe²⁺,that is the area strength of the spectrum with the isomeric shift notless than 0.8 mm/sec and not larger than 1.5 mm/sec:

TABLE 1 sintering temperature A/B 320° C. sample 6 0.77 400° C. sample 20.34 600° C. sample 1 0.15

In Experiment 1, if X-ray diffraction is carried out on samples 1, 2 and6, no spectrum proper to a phase containing Fe²⁺, for example, trivalentiron compounds, was observed, as shown in FIG. 2. However, if theMoessbauer spectroscopic measurement is performed on samples 1, 2 and 6,existence of the Fe³⁺-containing phase was confirmed. This is due to thefact that X-ray diffraction occurs only as a result of long-distanceinterference of crystals, whereas the Moessbauer spectroscope directlydetects the information in the vicinity of the atomic nuclei.

From Table 1, it is seen that the sample 6, with a sintering temperatureas low as 320° C., contains a larger quantity of a phase containing Fe³⁺not having long-distance order.

From Table 1, it is seen that A/B depends on the sintering temperatureof LiFePO₄, and that, the lower the sintering temperature, the more isthe content of Fe³⁺ in LiFePO₄.

If A/B shown in Table 1 is compared to FIG. 3 showing the relationbetween the sintering temperature of LiFePO₄ and the dischargingcapacity, it is seen that the smaller the value of A/B, that is thesmaller the amount of the trivalent iron compound containing Fe³⁺ inLiFePO₄, the higher is the capacity of the lithium ion secondarybattery. It is also seen that if LiFePO₄ is synthesized at a sinteringtemperature not lower than 400° C., the value of A/B is less than 0.3,thus realizing a high capacity.

So, it may be seen that, if LiFePO₄ with A/B equal to 0.3 is used as apositive electrode active material, a lithium ion secondary battery ofhigh capacity may be achieved.

INDUSTRIAL APPLICABILITY

As will be apparent from the foregoing description, the positiveelectrode active material according to the present invention contains acompound represented by the general formula Li_(x)M_(y)PO₄, where 0<x≦2and 0.8≦y≦1.2, with Md containing a 3d transition metal. Moreover,Li_(x)M_(y)PO₄ includes that with the BET specific surface area not lessthan 0.5 m²/g. This positive electrode active material, if used in anon-aqueous electrolyte secondary battery, realizes an extremely highcapacity.

Moreover, the positive electrode active material according to thepresent invention contains a compound represented by the general formulaLi_(x)(Fe_(y)M_(1−y))PO₄, where 0.9≦x≦1.1, 0<y≦1, with M containing a 3dtransition metal. With Li_(x)(Fe_(y)M_(1−y))PO₄, the ratio of A/B, whereA and B denote area strengths of the spectrum obtained with theMoessbauer spectroscopic method, is less than 0.3. This positiveelectrode active material, if used in a non-aqueous electrolytesecondary battery, realizes an extremely high capacity.

On the other hand, the non-aqueous electrolyte secondary batteryaccording to the present invention has a large capacity and an extremelyhigh capacity by employing LiFePO₄, obtained by prescribing thesintering temperature and the particle shape, as a positive electrodeactive material.

On the other hand, the non-aqueous electrolyte secondary batteryaccording to the present invention has a high capacity by employingLiFePO₄, with A/B less than 0.3, as a positive electrode activematerial.

Moreover, in the manufacturing method for the positive electrode activematerial according to the present invention, impurity-free single-phaseLi_(x)M_(y)PO₄ is obtained, thus realizing a high capacity surpassing120 mAh/g of a conventional non-aqueous electrolyte secondary battery.

1. A method for producing a positive electrode active materialcomprising: a mixing step of mixing a starting materials for synthesisof a compound represented by the general formula Li_(x)M_(y)PO₄, where0<x≦2 and 0.8≦y≦1.2, with M containing a 3d transition metal; and asintering step of sintering and reacting said precursor obtained in saidmixing step; wherein, in said sintering step, said precursor is sinteredat a temperature not lower than 400° C. and not higher than 700° C. 2.The method for producing a positive electrode active material accordingto claim 1 wherein, in said sintering step, said precursor is sinteredat a temperature not lower than 400° C. and not higher than 600° C. 3.The method for producing a positive electrode active material accordingto claim 1 wherein said Li_(x)M_(y)PO₄ is LiFePO₄.